The invention serves for the washing of heat exchangers, including for example air pre-heaters of the type Ljungstrom or Nowden, which consist of a rotating drum formed by crowns of heat exchanging sheets which, by passing alternatively through the burnt gas discharged and the combustion air newly fed, give to said air the heat previously removed from said burnt gases.
It is known that the sheet pack distributed in annular crowns must be periodically cleaned by both blowing with steam, compressed air, and washing with water. To this end it is known to use stationary or removable lances allowing said cleaning. Said lances comprise a plurality of nozzles directed downwards in order to direct the cleaning fluid used into the spaces existing between the sheets said heat exchanger consists of. The known lances of this kind have however some inconveniences:
(A) AS SAID LANCES GENERALLY HAVE A CONSTANT SECTION, OBVIOUSLY A PRESSURE GRADIENT OCCURS FROM THE ROOT TO THE END OF THE LANCE, CONSEQUENTLY THE FLUID JET COMING FROM THE NOZZLES ARRANGED NEAR THE EXCHANGER PERIPHERY ARE MORE EFFICIENT;
(B) AS THE SURFACE TO BE CLEANED IS LARGER IN THE PERIPHERY THAN NEAR THE EXCHANGER CENTER, A TIME OF CLEANING PROPORTIONAL TO THE AREA OF THE OUTERMOST CROWN OF THE EXCHANGER MUST BE USED.
On the other hand, it is not possible to make a conduit with a section decreasing from the periphery towards the center of said exchanger, as its size and cost would be excessive; nor to arrange on a single lance the nozzles in the radial direction of the exchanger, as that too would require an excessive size and cost.